An advanced photon counting imaging device for single x-ray counting is disclosed in U.S. Pat. No. 7,514,688 B2 (aus WO 2004/064168 A1) and is directed to increasing the time resolution capabilities of a photon counting imaging device. This is achieved by a photon counting imaging device for single x-ray counting, including a layer of photosensitive material, a source of bias potential, a source of threshold voltage supply, an N×M array of photodetector elements formed using the layer of photosensitive material, each photodetector element having a bias potential interface and a photodetector element output interface, the bias potential interface of each photodetector element being connected to the bias potential, an N×M array of high-gain low-noise readout unit cells, one readout unit cell for each photodetector element, each readout unit cell including an input interface connected to the photodetector element output interface, a high-gain voltage amplifying unit including a discriminator unit, and a digital counter unit including a digital counter and a digital counter output interface connected in series, each digital counter unit counting an output signal of the discriminator unit, the output signal being proportional to a number of electron-hole pairs generated by a photon in the respective photodetector element, a multiplexing unit including a row select and a column select circuit allowing to access each readout unit cell, to read out the digital data as actually stored in the digital counter to the digital counter output interface, each digital counter output interface connected to an output bus, and the output bus being connected to a data processing unit controlling the multiplexing unit.
The drawback of this device is to be seen in the fact that this device and its counting can become paralyzed. The counter is basically triggered by the edge of the discriminator output signal and can therefore become paralyzed under certain circumstances. This will occur whenever the discriminator output signal representing an amplified signal of the electron-hole pairs generated by an incident photon or a number of incident photons is overlapping a former discriminator output signal. Since every discriminator output signal pulse is being counted, the individual single photon pulses are no longer correctly detectable or countable as single pulses once they start to merge into each other. Although the device is capable of counting single such pulses, it will not be able to count such pulses correctly if they come too close in sequence. In detail: At a high photon flux, pile-up of incoming charge pulses can paralyze the photon counting detector during the time the individual pulses overlap. Generally, a charge pulse is detected if it causes the discriminator input signal to cross and exceed the discriminator threshold level. This threshold transition causes a counter increment thus counting the individual charge pulses. At very high photon flux, the charge pulses may pile up such that the discriminator input signal continuously exceeds the threshold level. As a result, only the first charge pulse is counted and the other charge pulses included in the pile-up are not counted due to the missing threshold transitions. Therefore, the counting is paralyzed and the observed count rate deviates from the incoming photon rate. The higher the incoming photon rate, the more pulses are overlapping and the higher the deviation of the observed count rate due to the paralysis. At very high rates, the observed count rate is even decreasing with increasing photon rate and finally results in total paralysis with zero observed count rate. This characteristic shows an ambiguous relation between the observed count rate and the incoming photon rate and makes a well-defined count rate correction impossible. U.S. Pat. No. 7,514,688 B2 (WO 2004/064168 A1) does not address the problem of counting loss due to pulse pile-up. This is the starting point of the present invention and its aim is to further improve the photon counting performance by avoiding the counting loss and the paralyzing effect which occurs when too many photons are coming in and cause the pulse pile-up problem.
EP 0 144 674 B1 discloses a method for correcting count rate losses of radiation events. Such losses can occur due to detector dead times during an acquisition time. It is suggested to a) detect radiation events (Z) which are subject to dead time losses during the acquisition time to obtain a succession of radiation detector event triggers, then b) provide for each detected radiation event a dead time signal (DT) the width of which corresponds to the dead time generated by the detected radiation event (Z), then c) subdivide the acquisition time into a succession of evaluation time intervals (ETI). In the course of the execution of such a method, it is suggested to further d) measure the fractional amount (DT′) of dead time within each evaluation time interval to obtain a measure for count rate losses, then e) evaluate from the fractional amount (DT′) of dead time a replication probability r, depending on the replication number m that is only so large as to make the replication probability r be less than one for any particular fractional amount (DT′) of dead time. Then, eventually, evaluate all radiation events detected between the end of one evaluation time and the end of a following one to generate f1) one pulse (P) for each detected radiation event between the ends of the succeeding evaluation times, and f2) a sequence of a number of pulses (P, P′, P″) for randomly selected events which selection corresponds to the replication probability r and which pulse number corresponds to the replication number m. To summarize, the problem of counting loss due to pulse pile-up is addressed in the following way: The exposure time is subdivided into successive evaluation time intervals; a measure for the pile-up probability in an evaluation time interval is determined by measuring the sum of all pulse widths by means of a separate time counting clock that has a high frequency and that is asynchronous to the pulses; additional counting pulses are randomly replicated in the following evaluation time interval according to the measure for the pile-up probability in order to compensate for the counting loss in the previous evaluation time interval. This approach has the following fundamental drawbacks: The continuous exposure time has to be subdivided into separate evaluation time intervals; the counting loss compensation is performed per evaluation time interval and is based on a measure determined in a previous evaluation time interval; the counting loss compensation is estimated and is based on an average measure for pile-ups and is not performed on a per event basis, thus only statistically corrects the count rate; a separate high-frequency clock is required for time counting; the time counting clock is asynchronous to the pulse events and introduces quantization noise to the time measurement; the pulse replication is randomly generated and not tied to the corresponding pile-up event.
US 2006/0276706 A1 discloses a medical imaging system. The system includes an input circuit configured to receive a voltage level signal indicative of a stream of pulses, a voltage level signal shape analyzer configured to determine shape characteristics of the received voltage level signal and an amount of time that the received voltage level signal matches a predetermined shape. A further counting circuit is configured to determine a true number of pulses from the shape characteristics and the amount of time. In other words, the problem of counting loss due to pulse pile-up is addressed in the following way: The pulse width of each pulse event is measured by means of a separate time counting clock that has a high frequency and that is asynchronous to the pulses; the measured pulse width is used to estimate the number of incoming pulses for each measured pulse. This approach has the following fundamental drawbacks: A separate high-frequency clock is required for time counting; the time counting clock is asynchronous to the pulse events and introduces quantization noise to the time measurement; a complex circuitry is required to generate the estimated number of pulses from the measured time value.
None of the above mentioned documents teaches a true solution to the pulse pile-up problem that can be reliably implemented in real photon counting imaging devices with stringent area and complexity limitations in the cells of the detector array. The problem of counting loss due to pulse pile-up can be described as follows: The counter is basically triggered by the edge of the discriminator output signal and can therefore become paralyzed under certain circumstances. This will occur whenever the discriminator output signal representing an amplified signal of the electron-hole pairs generated by an incident photon or a number of incident photons is overlapping a former discriminator output signal. Since every discriminator output signal pulse is being counted, the individual single photon pulses are no longer correctly detectable or countable as single pulses once they start to merge into each other. It is therefore the aim of the present invention to increase the performance and the reliability of a complete photon counting imaging device by reliably avoiding such paralysis and by improving the high-rate counting performance and the count rate correction means.